The present invention relates to a thread coupling for use in a drill string adapted for well drilling in earth and rock formations. The coupling includes at least one male thread and one female thread. The male thread is arranged upon a spigot that constitutes an integral part of a first drill string element. That drill string element includes a radial shoulder located at the inner end of the spigot, the shoulder having a contact surface directed towards the free end of the spigot. The female thread is connected to a second drill string element with an internal or bottom stop surface being provided at the inner end, or bottom, of the female thread.
Contact between the free end of the spigot and the bottom stop surface is hereinafter referred to as a "bottom stop", and contact between the radial shoulder and the free end of the second drill string element is hereinafter referred to as a "shoulder stop".
In a conventional thread coupling between two elements of a rock drilling string for percussive drilling, a bottom stop is normally used, e.g., see Fischer et al U.S. Pat. No. 3,537,738 disclosing coupling sleeves integral with one of the drill rods and Johansson et al U.S. Pat. No. 3,645,570 disclosing a separate coupling sleeve.
In cases where a female thread is formed in a coupling sleeve which is integral with the second drill string element, a bottom stop occurs as the free end of the spigot of one drill rod abuts the bottom of the integral coupling sleeve. In the case where the sleeve is separately attached to the second drill string element, a bottom stop occurs when the ends of the two drill rods abut each other within the sleeve.
For a coupling between a drill rod and a drill bit for percussive drilling, bottom stops are normally used. This is exemplified in Jones U.S. Pat. No. 1,959,919.
So-called shoulder stops in thread couplings between two elements in a rock drilling equipment for percussive drilling are also known. This is exemplified in Thurston U.S. Pat. No. 1,477,855. In this type of thread coupling a deterioration of the energy transfer can occur and also extensive local stresses can be present at the shoulder stop and/or at the bottom of the internal thread.
PCT Published Application No. WO87/04487 filed Jan. 14, 1987, discloses a percussive drill wherein a spigot of a drill rod engages a bottom of a female thread in a drill bit to form a bottom stop therewith. The shoulder on the drill rod is disclosed as being spaced from the free end of the female thread. In practice, however, that spacing, or gap, is gradually eliminated, whereupon there is created a shoulder stop in combination with the bottom stop.
This latter condition, i.e., wherein both a bottom stop and a shoulder stop are formed, is the most ideal condition for the transference of forces from one drill string element to the other. However, it is most difficult, due to normal manufacturing tolerances, to be able to manufacture the threaded coupling so that such an ideal condition exists immediately upon manufacture. Thus, for example, it may occur that the dimensions of the male and female thread components result in the formation of only a bottom stop, as in the above-referenced PCT Application for example. In such a case, however, it may occur that the threads are damaged as the result of bending moments applied to the drill string elements during a drilling operation, such damage being particularly apt to occur to the male threads.